The present invention is generically directed on a technique according to which acoustical signals are received by at least two acoustical/electrical converters as e.g. by multidirectional microphones, respective output signals of such converters are electronically computed by an electronic transducer unit so as to generate an output signal which represents the acoustical signals weighted by a spatial characteristic of amplification. Thus, the output signal represents the received acoustical signal weighted by the spatial amplification characteristic as if reception of the acoustical signals had been done by means of e.g. an antenna with an according reception lobe or beam. Thus, the present invention is generically directed on an electronically preset, possibly electronically adjusted and tailored reception xe2x80x9clobexe2x80x9d.
FIG. 1 most generically shows such known technique for such xe2x80x9cbeam formingxe2x80x9d on acoustical signals. Thereby, at least two multidirectional acoustical/electrical converters 2a and 2b are provided, which bothxe2x80x94per sexe2x80x94convert acoustical signal irrespective of their impinging direction xcex8 and thus substantially unweighted with respect to impinging direction xcex8 into first and second electrical output signals A1 and A2. The output signals A1 and A2 are fed to an electronic transducer unit 3 which generates from the input signals A1, A2 an output signal Ar. As shown within the block of unit 3 the signals A1,2 are treated to result in the result signal Ar which represents either of A1 or A2, but additionally weighted by the spatial amplification function F1(xcex8). Thus, acoustic signals may selectively be amplified dependent from the fact under which spatial angle xcex8 they impinge, i.e. under which spatial angle the transducer arrangement 2a, 2bxe2x80x9cseesxe2x80x9d an acoustical source. Thereby, such known approach is strictly bound to the physical location and intrinsic xe2x80x9clobexe2x80x9d of the converters as provided.
One approach to perform signal processing within transducer unit 3 shall be exemplified with the help of FIG. 2. Thereby, all such approaches are based on the fact that due to a predetermined mutual physical distance pp, of the two converters 2a and 2b, there occurs a time-lag dt between reception of an acoustical signal at the converters 2a, 2b.
Considering a single frequencyxe2x80x94xcfx89xe2x80x94acoustical signal, received by the converter 2a, this converter will generate an output signal
A1=Axc2x7sin xcfx89t,xe2x80x83xe2x80x83(1)
whereas the second transducer 2b will generate an output signal according to
A2=Axc2x7sin xcfx89(t+dt),xe2x80x83xe2x80x83(2)
whereat dt is given by                     dt        =                                            p              p                        ⁢            sin            ⁢                          xe2x80x83                        ⁢            θ                    c                                    (        3        )            
therein, c is the sound velocity.
By time-delaying e.g. A1 by an amount
xcfx84=pp/cxe2x80x83xe2x80x83(4)
and forming the result signal Ar from the difference of time-delayed signal A1xe2x80x2xe2x80x94as a third signalxe2x80x94namely from
A1xe2x80x2=Axc2x7sin xcfx89(t+xcfx84), andxe2x80x83xe2x80x83(5)
A2=Axc2x7sin xcfx89(t+dt),xe2x80x83xe2x80x83(2),
there results, considered at the frequency xcfx89, a spatially cardoid weighted output signal Ar as shown in the block of transducer unit 3:
|Ar|=|A1xe2x80x2xe2x88x92A2|=2A sin(xcfx89(xcfx84xe2x88x92dt)/2)=2A sin(xcfx89(xcfx84xe2x88x92pp*sin xcex8/c)/2).xe2x80x83xe2x80x83(6)
At xcex8=90xc2x0 Ar becomes zero and
at xcex8=xe2x88x9290xc2x0 Ar becomes
Armax=2A sin xcfx89pp/c.xe2x80x83xe2x80x83(7)
Such processing of the output signals of two omnidirectional order converters leads to a first order cardoid weighing function F1(xcex8) as shown in FIG. 3. By respectively selecting converters with higher order acoustical to electrical conversion characteristic i.e. xe2x80x9clobexe2x80x9d and/or by using more than two converters, higher orderxe2x80x94mxe2x80x94weighing functions Fm(xcex8) may be realised.
In FIG. 4 there is shown the amplitude Armax-characteristic, resulting from first order cardoid weighing as a function of frequency f=xcfx89/2xcfx80. Additionally, the respective function for a second order cardoid weighing function F2(xcex8) is shown. Thereby, there is selected a physical distance pp of the two converters 2a and 2b of FIG. 1 to be 12 mm.
As may clearly be seen at a frequency fr which is
fr=c/(4pp)xe2x80x83xe2x80x83(8)
maximum amplification occurs of +6 dB at the first order cardoid and of +12 dB at a second order cardoid. For pp=12 mm, fr is about 7 kHz.
From FIG. 4 a significant roll-off for low and high frequencies with respect to fr is recognised, i.e. a significant decrease of amplification.
Techniques for such or similar type of beam forming are e.g. known from the U.S. Pat. No. 4,333,170xe2x80x94acoustical source detectionxe2x80x94, from the European patent application 0 381 498 directional microphonexe2x80x94or from Norio Koike et al., xe2x80x9cVerification of the Possibility of Separation of Sound Source Direction via a Pair of Pressure Microphonesxe2x80x9d, Electronics and Communications in Japan, Part 3, Vol. 77, No. 5, 1994, page 68 to 75.
Irrespective of the prior art techniques used for such beam forming with at least two converters, the distance pp is an important entity as may be seen e.g. from formula (8) and directly determines the resulting amplification/angle dependency.
Formula (8) may be of no special handicap if such a technique is used for narrow band signal detection or if no serious limits are encountered for geometrically providing the at least two converters at a large mutual physical distance pp.
Nevertheless, and especially for hearing aid applications, the fact that fr is inversely proportional to the physical distance pp of the transducers is a serious drawback, due to the fact that for hearing aid applications the audio frequency band up to about 4 kHz for speech recognition should be detectable by the at least two transducers which further should be mounted with the shortest possible mutual distance pp. These two requirements are in contradiction: The lower fr shall be realised, the larger will be the distance pp required.
It is thus a first object of the present invention to remedy the drawbacks encountered with respect to pp-dependency of known acoustical xe2x80x9cbeam formingxe2x80x9d.
The first object of the present invention is reached by providing a method for electronically selecting the dependency of an electric output signal of an electronic transducer unit from spatial direction wherefrom acoustical signals impinge on at least a first and a second acoustical/electrical converter, connected to the inputs of said transducer unit, thereby inputting first and second electric signals thereto, which comprises the steps of
generating at least one third electric signal in dependency from mutual phasing of the first and the second electric signals, said phasing being multiplied by a constant or a frequency-dependent factor and further from a fourth electric signal which depends from at least one of the first and the second electric signals;
generating the output signals of the transducer unit in dependency of the third signal and further from a fifth electric signal which is dependent from at least one of the first and the second electric signals.
Thereby, it becomes possible, irrespective of the actual physical mutual distance of the two converters, to select said dependency, thereby pre-selecting same and possibly tuning and adjusting same, to result in a dependency as if the at least two converters were physically arranged at completely different physical positions than they really are.
In a first preferred manner of realising the inventive method the fourth electric signal is selected to be linearly dependent only from one of the first and second electric signals, thereby being preferably directly formed by such first or second electric signal.
Nevertheless, in a today""s more preferred manner of realising the inventive method, the fourth electric signal is dependent on both first and second electric signals. In a preferred form the fourth electric signal has a predetermined or adjustable xe2x80x9clobexe2x80x9d characteristic, i.e. dependency from spatial impinging direction. Thereby in a preferred form of xe2x80x9clobexe2x80x9d realisation the fourth electric signal is generated by delaying one of the first and second signals and then summing the delayed signal and the other, undelayed signal of said first and second signals. Thereby, the fourth electric signal per se has an amplification to impinging angle dependency and thus definesxe2x80x94as was saidxe2x80x94for a xe2x80x9clobexe2x80x9d, as an example according to a dependency as was discussed with the help of the FIGS. 1 to 4.
In a further preferred form of realising the inventive method, either per se or combined with either method to generate the fourth signal as just stated, and especially combined with generating the fourth signal with a xe2x80x9clobexe2x80x9d-characteristic, it is proposed to generate the fifth electric signal in direct or linear dependency of at least one of the first and second electric signals, thereby preferably using one the said first and second electric signals as the fifth electric signal.
Thereby, and again per se or combined with either method of generating the fourth electric signal, especially combined with generating the fourth electric signal with a xe2x80x9clobexe2x80x9d-dependency, it is proposed to generate the fifth electric signal as well with a xe2x80x9clobexe2x80x9d dependency from spatial impinging angle, which is realised in a first form by delaying one of the first and second signals and summing the delayed signal and the other of said first and second signals. Thereby, it becomes clear that the fourth electric signal, generated to define for a xe2x80x9clobexe2x80x9d characteristic, may directly be used as the fifth electric signal, having then the same xe2x80x9clobexe2x80x9d-characteristic.
In a further, clearly preferred realisation form of the inventive method and combined with any of the preferred realisation forms stated up to now and throughout the further description, it is proposed to generate the first and second electric signals in their respective spectral representation, thereby generating the at least one third electric signal in dependency of mutual phasing of respective spectral components of the first and second signals and multiplied by a constant frequency-independent or by frequency-dependent factors.
In a further preferred mode of operation, the frequency-dependent multiplication factors are selected to be inversely proportional to frequency, at least in a first approximation.
With an eye specifically on hearing aid applications, wherefore the present method is most suited, but may be clearly applied to others, it is proposed that the real physical distance of the first and second converters to be at most 20 mm, whereby the virtual distance, which is at least dependent from the phasing multiplication factor, is selected to be larger than the mutual physical distance of the two converters, in other words dependency of the transducer unit""s output signal from spatial angle becomes so as if, physically, converters were provided at considerably larger mutual distances than they really are. It goes without saying, that such technique is of very high advantage in any space-restricted applications, as especially in hearing aid applications.
To resolve the object mentioned above and to realise especially a hearing aid, whereat, irrespective of the physical position of at least two acoustical/electrical converters, a desired reception lobe may be tailored and possibly adjusted according to the needs, is realised inventively by an acoustical/electrical transducer apparatus comprising at least two acoustical/electrical converters spaced from each other by a predetermined physical distance, whereby the at least two converters generate, respectively, first and second electrical output signals and wherein the outputs of said acoustical/electrical converters are operationally connected to an electronic transducer unit, which generates an output signal dependent from said first and second output signals of said converters by an amplification function which function is dependent from spatial angle under which said converters receive acoustical signals, comprising:
a phase difference detection unit, the inputs thereof being operationally connected to the outputs of said converters and generating at its output a phase difference-dependent signal,
a phase processing unit, one input thereof being operationally connected to the output of said phase difference-detection unit, at least one second input of said processing unit being operationally connected to a factor-value-selecting source, a third input of said phase processing unit being operationally connected to at least one of the inputs of said at least two converters, said phase processing unit generating an output signal at its output according to a signal at said third input with a phasing according to a signal at said one input and at said at least one second input,
a beam-former processing unit with at least two inputs, one input being operationally connected to the output of said phase-processing unit, the second input being operationally connected to at least one output of said at least two con- verters.
Under all the aspects of the invention there is thus possible to realise
pV greater than pp.xe2x80x83xe2x80x83(9)
This especially for low-space applications, as especially for hearing aid applications.
Thereby, there is introduced the virtual distance pV of transducers, i.e. the distance of converters which would have to be physically realised to get an angle dependency as realised inventively.
Thereby, according to formula (8), fr may be shifted to lower frequencies:
It becomes possible to realise fr values well in the audiofrequency band for speech recognition ( less than 4 kHz) with physical distances of microphones, which are considerably smaller than this was possible up to now.
Multiplying the phase difference by a constant factor does nevertheless not affect the roll-off according to FIG. 4. This roll-off is significantly improved, leading to an enlarged frequency band Br according to FIG. 4 ifxe2x80x94as was saidxe2x80x94the predetermined function of frequency is selected as a function which is at least in a first approximation inversely proportional to the frequency of the acoustic signal.
For instance for the first order cardoid according to FIG. 3 and FIG. 4, there may be reached a flat frequency characteristic between 0,5 and 4 kHz and thus a significantly enlarged frequency band Br with well-defined roll-offs of amplification at lower and higher frequencies by accordingly selecting the frequency dependent function to be multiplied with the phase difference.